While on the highway last week, my wife's 2011 dash indicating emission fluid quality poor and a check engine light also came on. I didn't notice any degradation of performance... I figured I might be out of DEF so I tried to add some. It was already full. I took it to a local shop who indicated that DEF fluid will go bad just setting in a container at an auto-parts store or in her truck's DEF tank. None of the suppliers add an expiration date to their DEF containers. 300+ bucks later after shop drain old DEF, replaced with new DEF and reset codes, and driving for about 30 minutes, the codes and warnings returned. There is no apparent loss in performance. What could it be???